Vehicles, and particularly automotive vehicles which have anti-brake lock or anti-skid systems usually include hydraulic or other pressure fluid operated brakes in which, in dependence on operating conditions and braking conditions of the vehicle, braking pressure is controlled as a function of wheel acceleration or deceleration. Customarily, braking pressure can be dropped, maintained constant, or increased by suitable control of valves which drain pressurized braking fluid, block transmission and drainage of pressurized braking fluid, or permit application of pressurized braking fluid to the brakes. In accordance with known systems, braking pressure is first increased and, as slippage or excessive deceleration of a wheel with respect to a road surface is sensed, the braking pressure is then not increased but maintained at a constant level. If the slippage increases, that is, if the wheel continues to decelerate in accordance with a predetermined deceleration function, braking pressure is dropped or decreased. Decrease of braking pressure is usually continued if a predetermined deceleration threshold of the wheel is exceeded. The phase of braking pressure is continued until another predetermined deceleration threshold is passed, that is, until the wheel which has been controlled has a tendency to accelerate again.
Due to inertia effects within the system, and filters necessary to suppress disturbance and noise signals, accurate control in accordance with theoretically most desirable conditions is difficult. Particularly, it is difficult to determine the precise instant of time at which the braking pressure decrease should be terminated. Dead times and delay times which arise within the system thus may interfere with theoretically determined operation, and it may happen that the decrease in braking pressure is terminated too late.
It has been proposed to control the time during which braking pressure can be decreased, and to stop decrease of braking pressure after the elapse of a predetermined timing interval. It has also been proposed to permit braking pressure to become effective only if a further, and substantially higher, deceleration threshold is exceeded.
Decrease of braking pressure can be controlled by a retriggerable timing element--see, for example, the referenced German Patent Disclosure Document DE-OS No. 25 55 005 describing decreasing vehicle speed, in steps. The system is so arranged that a timing element is activated each time one of a predetermined speed intervals is passed, so that, overall, the time duration of pressure decrease will depend on the number of the speed intervals which have been passed during the deceleration of the wheel.
It has been found that such an arrangement does not provide for optimum adaptive control operation, since the overall duration of the control phase may become too long, because the pressure decrease phase of the control cycle may become excessively long. It is necessary to dimension the timing element with a sufficiently long timing constant, so that, if the timing element is only triggered once, a sufficient time to permit decrease of braking pressure will be available. This, initially comparatively long time, however, may be disadvantageous if, after the last speed interval has been passed, the remaining time interval of the element still has to elapse before a different braking pressure mode can result. Thus, the time delay of the timing element may become excessively long with respect to that which is actually desired.